1. Field of the Invention
The invention relates to a controller for a secondary battery.
2. Description of Related Art
In recent years, importance of each of a lithium-ion secondary battery, a nickel hydrogen battery, and other types of secondary batteries has been increased as an in-vehicle power supply. In particular, the lithium-ion secondary battery that can obtain high energy density in light weight has preferably been used as an in-vehicle high output power supply for a plug-in hybrid vehicle (PHV), an electric vehicle (EV), or the like.
The secondary battery of this type has such tendencies that a capacity thereof is significantly reduced when the battery remains unused in a high state of charge (SOC) and that an output characteristic thereof is deteriorated when the SOC is reduced. Accordingly, a lower limit SOC setting value (for example, 20%) that is higher than the SOC 0% and an upper limit SOC setting value (for example, 80%) that is lower than the SOC 100% are set. Then, the battery is used within a use potential width that is determined by the lower limit SOC setting value and the upper limit SOC setting value. In this way, high performance of the secondary battery is achieved. Japanese Patent Application Publication No. 2013-74706 (JP 2013-74706 A) can be raised as the related art on setting of an SOC use range of this type. In this publication, a vehicular controller is disclosed that controls an upper limit value of a battery capacity such that the upper limit value falls within a range, by which progress of deterioration can be suppressed, during parking of a vehicle and such that the secondary battery can be used in a wide range of the battery capacity so as to sufficiently secure traveling performance of the vehicle during traveling of the vehicle.
By the way, it has been known that the secondary battery, such as the lithium-ion secondary battery, is typically deteriorated with use. Main causes of the deterioration are precipitation of a charge carrier (lithium in the case of the lithium-ion secondary battery) in a negative electrode, formation of a film in the negative electrode, and the like. In the art disclosed in JP 2013-74706 A, even when progress of the deterioration as described above can be suppressed to some extent, the reduction in the usable battery capacity (and thus an energy amount) is possibly significant once the deterioration is progressed.